Gold making through multiboxing

Would it be a big advantage to multibox 3 characters to make gold?

[MiRai]

Sure.

Normally, if you can make X amount of gold per hour farming something with one character, then you should be able to make X * N amount of gold farming the same thing using more characters (N). It doesn't always translate linearly like that, but you're not going to lose gold by multiboxing multiple characters unless you're purposely trying to do so.

[Ualaa]

I did Dailies, with three different teams.
And invested the gold into buying herbs, to level Alchemy.
I had issues with 3 or 4 herb types, so had my own Herbalist for The Hinterlands, underwater (in Westfall) and in Eastern/Western Plaguelands.

Alchemy was chosen for daily transmutes.
This partners well with one or more Jewelcrafter toons to use the transmuted gems.
At the start of an expansion, five Jewelcrafters get patterns 5x quicker than a 1x boxer does.

You can do similar with Tailoring transmutes too.

Each expansion is different, but anytime there is a cooldown ability, stacking multiples is an advantage.

If you make Glyphs... multiple toons reduces your time to scribe them.
Same deal for prospecting ore.
Or Arbitrage on disenchanting greens/blues (determine the average price of the components, the average components yielded for a given green/blue's iLV, then buy anything that costs less than the value of the green/blue); enchanting mats don't cost anything to list, so you'll eventually get your gold.

In the WotLK expansion I had 37 alchemists doing transmutes.
That translated into 150K per week.